Nosov Magnitogorsk State Technical University (Magnitogorsk, Russia):

A. N. Zavalishchin, Dr. Eng., Prof., Dept. of Casting Processes and Materials Science, E-mail: zaval1313@mail.ru
O. N. Tulupov, Dr. Eng., Prof., Vice-Rector for Scientific and Innovative Work, E-mail: o.tulupov@mail.ru
M. I. Rumyantsev, Dr. Eng., Prof., Dept. of Materials Processing Technologies, E-mail: mikhail.rumyantsev54@bk.ru

Ausferr Research and Technology Center (Magnitogorsk, Russia):
E. V. Kozhevnikova, Company Engineer, E-mail: ausferr_elena@bk.ru

A. E. Volosenko, Mag. Eng., Dept. of Casting Processes and Materials Science, Nosov Magnitogorsk State Technical University, participated in this research.

Active development of pipeline transport of gas and oil with increasing working pressure to 120 atm. increases the need for pipes with large wall thickness that correspond the requirements of the DNV OS-F101 standard. Quality of continuously cast billets is decisive for improving quality of sheet metal for main pipelines. Inheritance of cast structure imperfections by a hot-rolled sheet leads to structural heterogeneity of the strip and the layered nature of the fracture surface and adversely affects the mechanical properties and corrosion resistance. Structural heterogeneity of rolled products is appeared in the form of axial ferrite-martensitic metal banding and metal banding in the main section of the sheet consisting of a mixture of ferritic and pearlitic grains — pearlitic metal banding. The flatness in the axial zone of the rolled products is due to axial chemical heterogeneity which is objectively formed during crystallization of the continuously cast billet and further phase transformations. The axial chemical inhomogeneity does not resolve despite the recrystallization of the structure and deformation and the high content of alloying elements contributes to the formation of the martensite phase and large carbonitride precipitates. The cause of pearlite bands is considered usually to be the presence of dendritic segregation. According to us the reason of this metal banding is the shift of the temperature front of γ → α transformation parallel to the sheet surface in depth as a result of which before the next volume of formed ferrite the concentration of carbon dissolved in austenite increases with the subsequent formation of pearlite. The enrichment of austenite proceeds along the boundaries preserved from the δ → γ transformation during cooling the slab and the formed pearlite structure repeats the shape of the boundaries of these grains in the section parallel to the sheet plane.

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